DVD is an optical storage medium with large data capacity. A single layer DVD can store about seven times more data than a standard CD while their physical dimensions including diameter and thickness are the same. The data on DVD is recorded in the form of microscopic pits and land along the spiral track, which is similar to CD; however, DVD has much smaller pit size with higher densification possibly on multiple layers. A DVD recording and reproducing system needs to provide a precise mechanism to record and reproduce these microscopic high density data signals. In addition, due to the popularity of CD, it needs to provide CD/DVD compatibility like all commercial DVD players and drives.
A DVD recording and reproducing system includes a spindle motor with a turn table configured to position and rotate an optical disc (DVD or CD), an optical pick-up unit configured to focus in on pits and trace a data track, and a control system for controlling the spindle motor and the optical pick-up unit.
The optical pick-up unit uses an objective lens to focus a laser beam onto the surface of the optical disc to record or reproduce data. Since the DVD has a small pit size with high densification, the laser beam needs to be focused in on small spot. To reduce the spot diameter, the optical pick-up unit for the DVD employs a laser diode with a reduced wavelength and an objective lens with a large numerical aperture.
To read data from an optical disc, the objective lens focuses a laser beam on the surface of the optical disc and a photo detector measures the intensity of the reflected light from the reflective layer of the optical disc. When it hits a pit in the data layer, the reflected light is more diffused, or has a weak intensity. When it strikes a smooth area, or land, the reflected light has a strong intensity. The signal processor identifies pits and land based on the measured light intensity, and converts them to an electrical signal, which is sent to a decoder.
Similarly, to record data on an optical disc, the objective lens focuses the light on the dye layer of the optical disc. The intensity of the laser beam emitted by a laser diode is varied according to a data signal transmitted to the laser via a data storage device. Depending on the intensity of the laser beam, the dye that is used to form the optical disc reacts to the heat and light to create a series of recorded pits and land.
Focusing and tracking errors caused by eccentricities in the rotation of the disk and vibration due to the rotation of the disk are detected by a signal processor using a light beam image formed in a photo detector. When an objective lens is closer to or farther away from an optical disc than the focal length of the objective lens, a signal processor can detect abnormality from a light beam image formed in the photodetector and produce a focus correction signal.
For tracking control, a three spot push-pull radial tracking method (U.S. Pat. No. 4,532,619 and U.S. Pat. No. 6,828,537) is usually employed, which utilizes a diffraction grating wherein the light is converted into a central peak and two side peaks. If the optical head is on track, the central beam is configured to be centered on the data track and two side beams are configured to be centered on land. A photo detector will measure a lower light intensity for a central spot than for side spots because the central beam is tracking on both pits and land while side beams are tracking on land. In addition, two side beams will have the same intensity. If the optical head is deviated from the data track, the central spot will have a higher light intensity and side spots will have lower intensities. Also, the intensities of two side spots will be different. In that case, the signal processor produces a tracking correction signal.
Conventional optical pick-up units are actuated by an electromagnetically driven servo system comprising coils and magnets, which usually provides two-axis controls; one for focusing control which drives the objective lens in the direction perpendicular to the optical disc surface and the other for tracking control which drives the optical head in the radial direction of the optical disc. The servo system includes a moving part that conducts macroscopic translational motions for focusing and tracking, and generates a certain degree of vibration.
The aforementioned conventional optical pick-up units show some limitations and drawbacks mainly due to their slow response time, a lack of degree of freedom, and complexity. Many DVDs use dual layers. The second layer can use either parallel track path layout or an opposite track path layout. The parallel track path layout in which both layers read from the center of the disc is suitable for a computer random access application such as DVD-Rom, a disc providing the choice between wide screen and full screen versions, or a disc having special DVD features along with a movie. The opposite track path layout in which the first layer reads from the inside to the outside while the second layer reads from the outside to the inside allows near continuous playback for long movies with a high picture quality. Both methods usually show noticeable time delay (freezing) for transition and refocusing from one layer to another because conventional optical pick-up devices use the macroscopic servo technologies to actuate controller, which results in a slow response time.
The tilt of an optical disc caused by the deflection and mechanical run-out of a disk, increases coma aberration, which is critical in systems with a large numerical aperture. Thus, tilt compensation is strongly required for DVD recording and reproducing systems. However, it is difficult for a two-axis optical pick-up actuator to compensate for coma aberration. Moreover, there is a sub-resonance problem due to mass unbalance or nonlinearity of the moving parts.
In order to solve those problems, optical pick-up units used in DVD recording and reproducing systems require three-axis motions for focusing, tracking, and tilt compensation. Such a device is described in US Patent No. 2004/0052180 A1, wherein a three-axis macroscopic mechanical servo system is used. However, the three-axis macroscopic actuator has many disadvantages, including, but not limited to, its complicated structure, large volume, heavy weight, low speed, high cost, and vulnerability to vibration.
Moreover, to provide the CD/DVD compatibility, it requires complicated structures such as an additional objective lens (U.S. Pat. No. 5,687,154), a holographic optical element (U.S. Pat. No. 5,717,674), or a liquid crystal shutter (U.S. Pat. No. 5,748,603).
Further, there is a growing need for high-speed, and small and thin size DVD recording and reproducing system. However, it is very difficult to satisfy the requirements for high-speed and/or miniaturization when the conventional servo technology is employed.
Thus, there is a practical need for an optical pick-up unit for DVD recording and reproducing systems that provides focusing, tracking, seamless layer jumping, tilt compensation, and CD/DVD compatibility with minimal macroscopic motion or even without macroscopic motion. This device also must be able to withstand vibration and satisfy the need for high-speed, miniaturization, and low production cost.